A rotating electrical machine such as a turbine generator using hydrogen gas as a cooling medium for cooling a coil and a structure inside a machine is provided with a sealing unit between a rotor shaft and a housing. The sealing unit seals hydrogen gas inside a housing or a rotating electrical machine with seal oil fed at a pressure higher than a pressure inside the housing. Seal oil is fed from a seal oil feeding apparatus to the sealing unit so as to always flow out from the sealing unit to the inside and outside of the housing. The seal oil feeding apparatus collects seal oil flowed out to the inside and outside of the housing, and feeds the oil back to the sealing unit.
Hydrogen gas mixes in seal oil flowed to the inside of the housing. The seal oil feeding apparatus is provided with a hydrogen detraining tank, a float trap tank, and an air detraining tank, as a separation unit for eliminating hydrogen gas. Seal oil flowed out to the inside of the housing is fed to the hydrogen detraining tank where most hydrogen bubbles are eliminated. Then, the seal oil is sent to the float trap tank. The float trap tank is provided with a float valve, which ejects seal oil to the air detraining tank when the liquid level of reserved seal oil exceeds a certain level.
The area from the sealing unit to the float value is kept at the same pressure as the inside of the housing, and the air detraining tank is opened to atmospheric pressure. The hydrogen detraining tank is placed at a position higher than the float trap tank so as to send out seal oil to the float trap tank. The hydrogen detraining tank is placed at a position higher than the air detraining tank so that seal oil flows out from the hydrogen detraining tank to the air detraining tank even if the internal pressure of the housing is lowered to atmospheric pressure.
The float trap tank is desirably placed at a height between the heights of the hydrogen detraining tank and the air detraining tank for flowing seal oil to the air detraining tank even if the inside of the housing is close to atmospheric pressure. However, a building should have sufficient height and installation space to have a sufficient height difference between the liquid levels of the tanks so that seal oil flows between the tanks. Since a pressure in the float trap tank becomes the same as an inside pressure of the housing, and seal oil can be flowed to the air detraining tank even if the float trap tank is placed at a position lower than the air detraining tank. Therefore, if the sufficient height difference is not provided within the building, the hydrogen detraining tank is merely placed at a position a little higher than the air detraining tank, and the float trap tank is placed at a position lower than the air detraining tank.
However, if the position of the float trap tank is lower than the air detraining tank, seal oil may flow backward from the air detraining tank when the pressure inside the housing of the rotating electrical machine decreases close to atmospheric pressure, and the float valve opens. The float valve closes only when the liquid level decreases. Therefore, the liquid level is difficult to confirm when the float flap tank is filled with seal oil. Hence, the liquid level of seal oil is unknown until seal oil overflows the hydrogen detraining tank and flows into the interior of the rotating electrical machine.